1. Field of the Invention
This invention relates to a pulse pattern generating apparatus that generates a test signal of a predetermined pattern for measuring the waveform quality of a digital signal by using plural digital-analog converters and then outputting the generated signal to a test subject, and particularly to a pulse pattern generating apparatus that outputs a test signal of high waveform quality even if the shape of an eye pattern is changed.
2. Description of the Related Art
When a digital signal using an electric signal or an optical signal is transmitted through a transmission line or inputted to a device, which is a test subject, the waveform quality of the digital signal is deteriorated by the characteristics of the test subject. The deterioration in the waveform quality causes increase in the bit error rate, increase of jitter, variance of the amplitude of waveform, change in the shape of an eye pattern and the like. Generally, for testing the deterioration in the waveform quality, a test signal is inputted to a test subject and an output signal outputted from the test subject is received. Then, the received signal and the test signal are compared with each other to measure the bit error rate (see, for example, JP-A-8-331102 (paragraph nos.0002-0008, FIGS. 3 and 4)) or to measure an eye pattern (see, for example, JP-A-2001-144819 (paragraph nos.0002-0008, FIG. 8)). The test is thus performed.
Therefore, the test signal needs to have high waveform quality (i.e., less jitter, less variance in amplitude, less noise, less overshoot/undershoot, high opening rate of the eye pattern and the like).
FIG. 1 shows the structure of a conventional pulse pattern generating apparatus.
FIG. 2 shows an eye pattern (with a cross point of 50%) of a test signal outputted from the apparatus shown in FIG. 1.
In FIG. 1, a voltage value setting unit 10 has a pattern generator circuit 11 and sets a voltage value for causing a signal outputted from the pattern generator circuit 11 to have an eye pattern of a predetermined shape. The pattern generator circuit 11 outputs an M-sequence pseudo-random pulse pattern (hereinafter simply referred to as pulse pattern) signal.
A waveform generator unit 20 has digital-analog converters (hereinafter simply referred to as DA converters) 21 to 24, an amplifier 25, an upper limit clipping circuit 26 and a lower limit clipping circuit 27. The waveform generator unit 20 generates and outputs a pulse pattern signal having an eye pattern of a desired shape in accordance with the setting from the voltage value setting unit 10. The pulse pattern signal outputted from the waveform generator unit 20 is a test signal of the predetermined pattern.
The DA converters 21 to 24 output voltage values set in accordance with the setting from the voltage value setting unit 10. The amplifier 25 amplifies the pulse pattern signal from the pattern generator circuit 11 in accordance with the output from the DA converter 21, and the outputs the amplified pulse pattern signal. The upper limit clipping circuit 26 includes, for example, a diode, a resistor, a capacitor and the like. The upper limit clipping circuit 26 clips the pulse pattern signal outputted from the amplifier 25 and offset by the output from the DA converter 22, at an upper limit value of a certain constant level in accordance with the output from the DA converter 23, and outputs the clipped pulse pattern signal. The lower limit clipping circuit 27 includes, for example, a diode, a resistor, a capacitor and the like. The lower limit clipping circuit 27 clips the pulse pattern signal outputted from the upper limit clipping circuit 26, at a lower limit value of a certain constant value in accordance with the output from the DA converter 24.
The operation of this apparatus will now be described.
The pulse pattern generator circuit 11 of the voltage value setting unit 10 outputs a pulse pattern signal of small amplitude to the amplifier 25. The voltage value setting unit 10 sets the voltage values of the DA converters 21 to 24 synchronously with the pulse pattern signal from the pulse pattern generator circuit 11. The DA converters 21 to 24 outputs the voltage values thus set.
Then, the amplifier 25 amplifies the pulse pattern signal of small amplitude to desired amplitude at an amplification factor corresponding to the voltage value outputted from the DA converter 21. The pulse pattern signal is amplified to amplitude that is sufficiently larger than the amplitude of the pulse pattern signal outputted from the lower limit clipping circuit 27.
The amplified pulse pattern signal is offset by the voltage value outputted from the DA converter 22 and inputted to the upper limit clipping circuit 26. For example, because of the offset from the DA converter 22, the lower limit value used for clipping by the lower value clipping circuit 27 becomes 1 [V].
Moreover, the upper limit clipping circuit 26 performs clipping at an upper limit value of a level corresponding to the voltage value outputted from the DA converter 23, for example, at a level slightly lower than a high level. Specifically, the level is determined as the output from the DA converter 23 is added as a bias voltage of a diode, not shown, of the upper limit clipping circuit 26. Then, the clipped pulse pattern signal is outputted to the lower value clipping circuit 27. Since the voltage of the level at which clipping is performed is changed by the bias voltage to the diode, the voltage value of the level at which clipping is performed and the voltage value outputted from the DA converter 23 have a nonlinear relation.
Then, the lower limit clipping circuit 27 performs clipping at a lower limit value of a level corresponding to the voltage value outputted from the DA converter 24. For example, it performs clipping at a position where the cross point of the eye pattern is 50% as shown in FIG. 2. Then, the pulse pattern signal shown in FIG. 2, which has the new levels at which clipping is performed by the clipping circuits 26 and 27, as its high and low levels, is outputted to a test subject, not shown. Of course, the level difference between the new high and low levels is the amplitude. Although the pulse pattern signal outputted from the pattern generator circuit 11 has overshoot and undershoot, these are eliminated by the clipping circuits 26 and 27. In this manner, the pulse pattern signal of high waveform quality is outputted. The offset in FIG. 2 is not the voltage value outputted from the DA converter 22 but is the level difference between the level of 0 [V] and an intermediate level between the low and high levels.
The apparatus shown in FIG. 1 as described above outputs a test signal of high waveform quality by setting the voltage values of the plural DA converters 21 to 24 in accordance with the pulse pattern signal. In this apparatus, the cross point and amplitude, which are parameters to determine the shape of the eye pattern, have fixed values. This is because the voltage values of the DA converters 21 to 24 are closely related with the shape of the eye pattern and cannot be set easily.
For example, the case of changing the value of the cross point from 50% to 30% while the amplitude remains unchanged will now be described with reference to FIGS. 3A to 3C. FIGS. 3A to 3C show the eye patterns of the pulse pattern signal. FIG. 3A shows the eye pattern of the pulse pattern signal outputted form the amplifier 25. FIG. 3B shows the eye pattern of the pulse pattern signal with the cross point of 50%. FIG. 3C shows the eye pattern of the pulse pattern signal outputted from the amplifier 25 (having a larger amplification factor than in FIG. 3A).
First, when the cross point is 50%, as described above, clipping is performed at level L1 of the upper limit value and at level L2 of the lower limit value as shown in FIG. 3A. Thus, a pulse pattern signal having the eye pattern with the amplitude shown in FIG. 3B and the cross point of 50% is generated.
On the other hand, in the case of changing the cross point to 30%, if level L2 of the lower limit value shown in FIG. 3A is simply raised, the amplitude changes. Moreover, the changed level and level L2 are different levels. Thus, the setting of the voltage value of the DA converter 21 is first changed and the amplification factor of the amplifier 25 is increased to generate a pulse pattern signal having the eye pattern shown in FIG. 3C. Then, the setting of the voltage values of the DA converters 23 and 24 is changed and clipping is performed at level L3 of the upper limit value and at level L4 of the lower limit value. Thus, a pulse pattern signal having the same amplitude as in FIG. 3B and a different cross point is generated. Of course, to equalize level L2 and level L4 of the lower limit value, the voltage value of the DA converter 22 is set and the quantity of offset is changed.
Similarly, in the case of changing the amplitude alone, it is necessary to not only change the setting of the voltage value of the DA converter 21, which determined the amplification factor of the amplifier 25, but also change the setting of the voltage values of the DA converters 22 to 24. Since the shape of the eye pattern is closely related with the voltage values set in the DA converters 21 to 24, the parameter values are generally fixed.
However, while users strongly want to input a pulse pattern signal having large amplitude or change the cross point depending on the test subject, there is a problem that the fixed shape of the eye pattern limits the subjects that can be tested. If a user personally changes the shape of the eye pattern, it is difficult to optimally set the voltage values of all the DA converters 21 to 24 and a pulse pattern signal of poor waveform quality is outputted. This causes a problem that the test cannot be carried out accurately. Particularly when the transmission rate of the pulse pattern signal is very high, for example, higher than 10 [Gbps], it is difficult to set the voltage values of the DA converters 21 to 24.